Packages formed with films are manufactured by a good number of methods which utilize to advantage the characteristics of the films such as the bag sealing method, twist wrapping method, thermal shrink wrapping method, cohesive wrapping method by use of specific films represented by Saran Wrap (product by Asahi-Dow Limited), stretch wrapping method, skin packing method intended to provide intimate adhesion between the film and its contents by virtue of heat and vacuum, vacuum packing method designed to produce tightly drawn packages by means of evacuation and the like. These methods require respective wrapping characteristics. For each packaging method, therefore, it is important to select a film whose basic material, composition, form and characteristic attributes best suit the wrapping characteristics of the particular method employed.
The films of this class, depending on various uses found therefor, are required to possess a broad spectrum of properties befitting an assortment of factors including the kind of article to be wrapped, the condition of preservation of the wrapped article and the like. Recently, owing to increasingly more exaction of special properties, the practice of using composite films formed of a multiplicity of layers of varying properties has come to find favorable acceptance. The multilayered film which is produced by preparing a substantially unoriented film or oriented film, then melt extruding other resin into a film and laminating the freshly extruded film with the former film offers one example.
A film of improved heat sealing property produced by fusion laminating an unoriented polypropylene layer of the cast method (called as "C.PP") or an oriented polypropylene layer (O.PP) with a layer of other resin and a film coated with a vinylidene chloride type latex or solvent to acquire a barrier property (called as "K-coat film") are other examples. These and other various types of films and their various combinations are selected to suit numerous applications.
On the other hand, a coextruded film which is produced by melting several different resins in some extruders and, with the aid of a multilayered die, joining the respective extruded sheets of the resins within the die, fusing the joined resin sheets, extruding the resultant composite sheet and cooling it into a film or sheet is widely known.
The optimum extruding and stretching conditions required for successful conversion of these multilayered sheets into highly oriented films vary from one to another of the resins used in the component layers making up the composite sheets. Owing to this variability of the optimal extruding and stretching conditions, the films under production suffer detestable phenomena such as inconsistent wall thickness, streaks, puncture, rupture, layer separation, and blushing due to coarsened interfaces, and the properties acquired by the produced films differ from those the films are expected to acquire. Elimination of these difficulties has turned out to be an extremely difficult task.
This invention aims to provide films which are usable for various purposes, particularly films which satisfy the purpose of shrink wrapping. Now the films of the kind produced by this invention specifically for the purpose of shrink wrapping will be described below by way of illustration of the invention.
Generally, the shrink wrapping method, on the principle of full use of the heat shrink property of a film which has been stretched to acquire a specifically set orientation, comprises the steps of loosely pre-wrapping or sealing a given article subjected to wrapping so as to enclose the article with a film and, thereafter, exposing the film to a heat medium such as current of hot air, infrared ray, hot water, and the like and thereby causing the film to shrink and come into tight contact with the overall contour of the article. This method is characterized by the fact that the produced package has beautiful appearance enough to add to the commodity value of the wrapped article, keeps the contents hygienic, permits shoppers to examine the quality of the contents by the senses of vision and touch, keeps the contents tightly in position no matter whether the article consists of a plurality of pieces or a single piece and provides the contents with ample protection against vibrations and impacts. Compared with the stretched wrapping method which is used extensively as in supermarkets, the shrink wrapping method enjoys high speed of wrapping work. The shrink wrapping method is now finding general acceptance with ever increasing impetus in the industrial packaging operations which generally involve articles too voluminous and heavy to be advantageously handled by the stretch wrapping method alone. This fact has come to attract keen attention of the industry.
Articles which are too irregular in contour to be packaged by the stretch wrapping method and articles which do not permit use of rigid auxiliary containers such as trays can be advantageously packaged by this shrink wrapping method. Further, this method enjoys the advantage that the article can be packaged with more tightness than by any other method. Despite these advantages, this method still has the disadvantage that the film wrapped around the article must be heated thoroughly until it amply shrinks.
The film currently used more extensively by this shrink wrapping method is a drawn film of plastic polyvinyl chloride (hereinafter referred to as "PVC") resin. The popular use of this film is ascribed to the film's outstanding ability to produce high degree of heat shrinkage at a relatively low temperature and provide advantageous shrink wrapping in a wide range of temperatures. On the other hand, this film is deficient in heat sealing property, preservability (due to inclination toward lose of orientation by the action of a plasticizer), and moistureproofness. It further entails hard problems such as the impairment of hygiene by the plasticizer, emission of chlorine type gas and other noxious gases during the fusion of film by the heated wire, liberation of corrosive noxious gases during the combustion of film in the incinerator, inferior resistivity of the film to cold weather and consequent inclination of the film toward rigidification, embrittlement and rupture.
As a result, increasing attention has come to be focussed on adoption of polypropylene (hereinafter referred to as "PP") type film for the shrink wrapping method in recent years. Unfortunately, the PP film is inferior in shrinkability to the PVC film. The drawn PP type film excels in mechanical properties, moistureproofness, heat seal strength, heat resistance, film modulus, and the like and, therefore, proves highly suitable for use as a shrink wrapping film.
It is also advantageous over the PVC film in respect that the cost of raw material and the specific gravity are both lower than those of the PVC film. Since PP is a rigid crystalline high polymer possessing a high softening point, the PP film shrink at higher temperatures than the conventional drawn films and exhibit a small shrink percentage at low temperatures in the neighborhood of 100.degree. C. In the process of shrink wrapping, therefore, the PP film must be heated at a higher temperature. Further because of sharp dependence of the shrink percentage upon temperature, the film in the course of wrapping undergoes locally uneven heating and entails uneven shrinkage which tends to induce such phenomena as "wrinkles" and "unshrinking spots" which are highly quite detestable from the viewpoint of practical use of the film. If the film is amply heated to preclude these phenomena, there inevitably ensues the serious disadvantage that the article being wrapped is exposed to excessive heating and the film itself loses transparency and sustain breakage in the sealed portion or around air vents. The PP film preponderantly comes in thin thickness (ex. 15-25.mu.). If the thickness is increased, the film gains in rigidity, tends to sustain breakage and consequently proves no longer suitable for wrapping.
The conventional polyethylene type film, in its unmodified form, cannot be given sufficient molecular orientation by drawing. The drawn film, therefore, exhibits low heat shrink percentage (particularly heat shrink stress), high shrink temperature, inferior strength and optical properties, and insufficient binding force on wrapped articles. Thus, the film produced in a thickness fairly large by the standard of ordinary wrapping films is adopted for limited, special applications.
The ordinary polyethylene type film which has its molecules thoroughly crosslinked by means of a high-energy ray and which has been amply drawn at a temperature exceeding the polymer's melting point enjoys high processibility, permits ready setting of molecular orientation by drawing in a wide temperature range, exhibits high heat shrink percentage and heat shrink stress, and excels in comparison with the ordinary polyethylene film in various properties such as optical properties including transparency and specular gloss, heat resistance, and the like. The film, however, shrinks at high temperatures and permits no easy heat sealing. When the film is subjected to shrink wrapping, therefore, it is degraded in strength, heat sealing property, and tear resistance and, consequently, is rendered susceptible to tear.
Further, the film suffers from problems such as great difficulty involved in the cutting and sealing of the film by use of an electric heat wire, degradation of physical properties, particularly optical properties, of the film in consequence of the heat shrinking treatment, degradation of film strength, infliction of rupture around the air vents in the film during the shrink wrapping, and ready formation of wrinkles in the film. The film, accordingly, suffers from low packaging speed and inferior package finish.
As implied above, one critical property the film is required to possess for successful shrink wrapping is the ability to permit satisfactory wrapping at low temperatures. This ability is particularly important when the film is used for wrapping fresh foodstuffs.
The manufacture of a drawn film of PP is accomplished by a method comprising the steps of melt extruding the polymer resin through an extruder die, quenching the extruded tubular sheet, reheating the cooled tubular sheet at a high temperature within the range of from 150.degree. to 160.degree. C., and forcing air into the inner cavity of the tubular sheet. In the case of a drawn film of low-density polyethylene, a similarly extruded tubular sheet of the polymer resin is biaxially drawn in an effort to set a high degree of molecular orientation in the film. In the course of the drawing, however, the sheet bursts, making the manufacture of film hardly practicable from the technical point of view.
Because of the difficulty, therefore, there is generally adopted a direct inflation method which comprises the steps of extruding the polymer resin at a temperature within the range of from 180.degree. to 220.degree. C., for example, and subsequently causing the extruded sheet, by means of a proper form of air, to be simultaneously cooled and inflated to a prescribed size.
This inflation method is characterized by being capable of producing the film inexpensively and very easily. The drawing effected in this method, however, causes disturbance and crystallization of the polymer molecules, degrades the film's optical properties, and fails to set the molecular orientation as desired. The film, therefore, has low heat shrink percentage and low heat shrink stress and shrinks at temperatures rather high by any standard. Hence, the film produced in a large thickness barely finds utility in limited special applications. With a view to preventing the disturbance of molecules and permitting the desired setting of thorough molecular orientation, there have been developed different methods, including one comprising the steps of molding the low-density polyethylene, then exposing the molded polymer to a high-energy radiant ray under a suitable set of conditions for thereby causing partial crosslinking of the molecules and, thereafter, reheating the molded polymer and drawing it at the elevated temperature. The film obtained by the conventional method, nevertheless, is not free from the aforementioned disadvantages in any case.
Many methods have been proposed for producing a film by inflating a multilayered sheet incorporating layers of different polyolefins or layers of both polyolefins and other polymers. For example, U.S. Pat. No. 3,682,767 discloses a film possessing improved melt strength and heat sealing property and exhibiting improved make-and-fill property in the wrapping of a liquid article, manufactured by the steps of mixing a copolymer of ethylene and an olefinically unsaturated monomer such as, for example, ethylene-vinyl acetate copolymer (hereinafter referred to as "EVA") with a linear copolymer of ethylene of a density of 0.93 to 0.96 g/cm.sup.3 and an .alpha.-olefin such as, for example, a modified high-density polyethylene (hereinafter referred to as "HDPE") and extruding the resultant mixture into a flat or tubular film. British Pat. No. 998,299 teaches a printable polyethylene film which is produced by the steps of mixing polyethylene such as low-density polyethylene (hereinafter referred to as "LDPE") or HDPE with EVA, subjecting the resultant mixture to a crosslinking treatment, and drawing the mixture into a film. And British Pat. No. 1,035,887 discloses a film excellent in low-temperature strength and other properties, which is produced by the steps of mixing LDPE with a linear medium-density polyethylene modified with a small amount of butene and drawing the mixture.
As concerns methods proposed heretofore for the manufacture of films, British Pat. No. 998,299 mentioned above discloses a method which comprises causing crosslinking of molecules in the aforementioned composition by subjecting the composition to a treatment with a peroxide or high-energy radiant ray, heating the sheet to a temperature around or higher than the melting point of polyethylene, and drawing the sheet as held at that temperature and British Pat. No. 992,897 discloses a method which comprises causing crosslinking of molecules in EVA by a treatment with a high-energy radiant ray, heating the sheet to an elevated temperature (preferably 100.degree. to 120.degree. C., for example), and drawing it at that elevated temperature. The films obtained by using these methods or compositions do not possess such outstanding optical properties, strength properties and low-temperature shrink properties as those obtained by the PVC film as described above, and they are also deficient in film-forming property and other similar properties.
Of the articles of the class which are packed by the shrink wrapping method, raw meat, processed meat, cheese, and other marine products and livestock products generally have quite irregular shape and are susceptible to deterioration due to the action of air. For protection against the deterioration, therefore, such an article is shrink wrapped by first vacuum packaging the article with an O.sub.2 -barrier film and shrinking the film with hot water for thereby causing the film to contract and come into tight contact with the overall contour of the article. The package thus obtained prevents possible leakage of meat juice or other liquid, preclude possible formation of wrinkles and occurrence of pinholes particularly in the barrier layer of the film and beautifies the outside appearance of the article contained. In the case of the package described above, since the film is exposed to direct contact with the food article which generally has been stored at normal room temperature or in a refrigerated space, the temperature of the film is not readily elevated and the shrinking treatment performed at a high temperature for a long time is detested as highly undesirable from the standpoint of the preservation of freshness of the food under treatment. For the shrink wrapping of such articles, therefore, need is felt for adoption of a film which shrinks quickly at a very low temperature and exhibits high shrink tension and heat sealing property. From the standpoint of workability, the film is required to possess modulus and cold resistance. A typical example of the conventional films which satisfy this requirement is offered by U.S. Pat. No. 3,741,253, which teaches a method comprising the steps of first preparing an innermost EVA layer in a tubular form, causing crosslinking of molecules in this layer with a treatment using a high-energy radiant ray, then coating this tubular layer with an O.sub.2 -barrier layer of vinylidene chloride polymer, subsequently fusion superposing another EVA layer on the coated tubular layer, and thereafter drawing the multilayered sheet at a temperature such as 88.degree. C. which causes no appreciable high molecular orientation of the vinylidene chloride type polymer and under conditions which cause no appreciable high molecular orientation of the other polymers used in the film in comparison with this invention. This film incorporated an O.sub.2 -barrier layer made of a vinylidene chloride type polymer. In this case, the method used for the manufacture of the film is so complicate as to render the quality control both difficult and expensive, the component layers of the film suffer from complicate thermal hysteresis and consequently are deprived of the effect of sudden cooling, and the drawing stability of the sheet is governed by the particular component EVA. If the drawing is carried out at a temperature falling short of 88.degree. C., for example, the sheet being drawing sustains puncture so readily as to render the drawing impracticable. Consequently, the shrink property of the film is degraded, the degrees of orientation of the component layers even including the vinylidene chloride type layer are lowered, and various other properties are likewise impaired. Besides, because of the crosslinking in the innermost layer, the film exhibits poor heat sealing property and becomes susceptible to curling. In addition, the film suffers from the phenomenon of layer separation in the course of the shrinking treatment. The vinylidene chloride layer, generally when it is quenched in the preparation of the raw sheet, assumes an amorphous rubbery state and then, with lapse of time, undergoes gradual crystallization. This crystallization tends to start within several minutes of the formation of the amorphous rubbery layer at the normal temperature. If, at this time, the layer is exposed to heat as from the lamination, it may possibly be deprived of the effect of quenching or the uniformity of texture. When the quenched layer is left to undergo crystallization in that case, it no longer exhibits elasticity and becomes brittle. This embrittlement tends to occur also when the drawing is effected at a very high temperature. Even when this layer is used as an inner layer in the film, a bend given to the film results in the formation of pinholes in this inner layer and the film, therefore, no longer retains its barrier property. Generally, for the vinylidene chloride type polymer to manifest its strength and other properties to advantage, it is extruded through a die directly into cold water (cooled to 8.degree. C. with ice, for example) to assume an amorphous rubbery texture. It cannot produce a film of well-balanced strength properties unless it is drawn at an extremely low temperature (such as 30.degree. C., for example). Under these conditions, the crystallization is generally accelerated by the orientation due to the drawing so that the crystallization is substantially completed in the course of the drawing and the molecular orientation is stably set by the end of the crystallization. When the drawing is carried out at a higher temperature, the molecular orientation in the vinylidene chloride layer begins to proceed gradually after the drawing of the other component layers of the film have been brought to completion. The film which is consequently obtained possesses a substantially low degree of molecular orientation and lacks strength, resistance to infliction of pinholes, and shrinking property. In this case, there is a wide gap between the temperature range in which the EVA resin (such as near melting point, for example) can be drawn and the optimum temperature range in which the vinylidene chloride resin (such as 30.degree.-35.degree. C., for example) can be drawn. After all, the temperature at which the film can be stably drawn falls around the aforementioned temperature range even when the temperature for effective drawing of the innermost layer is lowered, though to a slight extent, by means of crosslinkage and the proportion of the thickness of the innermost layer to the combined thickness of the remaining component layers of the film is substantially increased, say to 75%. The temperature may be decreased if the vinyl acetate content of the EVA polymer which is the principle layer was increased. The increased vinyl acetate content of the EVA polymer, however, brings about a serious disadvantage that the film is heavily degraded in shrinking property, shrink tension, heat-sealing property, heat resistance, strength, elastic modulus and the like and further impaired in time-course stability.